It is a common practice to provide a nuclear reactor so that the reactor safety vessel or containment is itself surrounded by a structure so that an annular compartment, chamber or space is defined between that structure and the containment, substantially all around the latter. An auxiliary equipment building can be provided adjacent this structure and the venting system can also include a stack or chimney for venting gases into the atmosphere. In general, a nuclear reactor of this type comprises a system for the depressurization of the containment as well as a system for maintaining a subatmospheric pressure, upon the occurrence of an event indicating a failure and which is provided with at least one blower connected to the stack or chimney.
In the past it has been customary, in addition, to provide the system for pressure relief of the containment with at least one metal-fiber filter and at least one molecular sieve downstream thereof.
The nuclear reactors to which these principles are applicable include pressurized-water reactors, boiling-water reactors and reactors of other types.
In general, the metal-fiber filter is provided within the containment vessel while the molecular sieve is provided in the auxiliary equipment building. It is, however, also possible to provide the metal-fiber filter in the auxiliary equipment building. An emergency filter which comes into play upon the occurrence of a failure of an "event" as described above, can also be provided in the auxiliary equipment building or at least in part in the annular space between the outer structure and the containment vessel.
The "failure" or "emergency" with which the invention is concerned is a so-called design failure in which radioactive substances may be liberated. The term, however, does not normally include so-called design-exceeding events which may involve melting of the core, i.e. a fuel or limited meltdown.
In design failures as well as with design-exceeding events involving melting of the core, radioactive substances are released by leakage processes from the containment vessel into the annular space. Prevailing viewpoints in the field of nuclear safety design the system for use of the emergency filter in the case of design failures. The design failures, however, release relatively small amounts of radioactivity by comparison to the design-exceeding events involving core melting
In the nuclear reactor system over which the invention is considered to be an improvement, the emergency filter is formed by at least one suspended-material filter, at least one active-coal filter and, optionally, at least one further suspended-matter filter downstream of the active-carbon filter.
The materials from which these filters are constructed are cellulosic fleeces, paper, coal and the like.
While these earlier system have been found to be satisfactory in most cases, they are not suitable for design-exceeding events involving melting of the core when one must maintain a suction action capable of removing radioactive components entering the space from the moment at which a melting of the core commences through depressurization and over the entire superatmospheric pressure phase in the safety vessel. In those situations, special and more complex means must be provided.